In stutterers it is hypothesized that both the left and right hemispheres of the brain compete to send signals to the muscles that produce speech. This causes conflict and interruptions in a stutterer's speech flow pattern. In contrast, under this hypothesis, in non-stutterers no such competition exists and brain signals produce speech only from one side of the brain (typically the left), thereby producing fluent speech patterns. At least one report submits that stuttering is caused by interrelated sensory and motor deficits. See e.g. J. Kalinowski et al., Effects Of Alterations In Auditory Feedback and Speech Rate On Stuttering Frequency, 36(1) Language and Speech, p. 1-16 (1993).
Conventionally, stuttering has been treated by several different types of treatment including psychiatric therapy, drug therapy, and electrical signals such as altered auditory feedback. Two types of altered auditory feedback which have been studied include delayed auditory feedback ("DAF") and introduction of a masking noise or masked auditory feedback ("MAF"). Generally described, DAF imposes a delay on the delivery of a feedback speech signal to a speaker/stutterer, while MAF serves to compete with a speaker's auditory feedback. It has been known for some time that DAF methods can be used to treat stuttering. See e.g. R. Pollock et al., A Solid State Delayed Auditory Feedback System, Biomedical Engineering, December 1976, pp. 413-414.
For example, M. E. Wingate in Stuttering: theory and treatment, p. 237 (Irvington 1976), describes a type of altered auditory feedback which can include DAF (delayed auditory feedback) to provide an emphasis on phonation, i.e., slowing down to extend syllable duration. However, this type of auditory feedback is thought to be achievable with or without the use of DAF as long as the same technique of syllable prolongation was employed. See e.g. W. H. Perkins, From Psychoanalysis to Discoordination, in H. H. Gregory (Ed.) Controversies about stuttering therapy, pp. 97-127 (University Press 1979). See also Andrew Stuart et al., Fluent Speech, Fast Articulatory Rate, and Delayed Auditory Feedback: Creating a Crisis For A Scientific Revolution?, 82 Perceptual and Motor Skills, p. 211-218 (1996).
In the past, slowed speech rates were found to be an important factor in the reduction of stuttering. For example, in W. H. Perkins et al., Phone rate and the effective planning time hypothesis of stuttering, 29 Jnl. of Speech and Hearing Research 747-755 (1979), the authors reported that stuttering was virtually eliminated when speakers reduced speech by approximately 75%.
U.S. Pat. No. 4,685,448 to Shames et al. discloses a vocal tactile feedback apparatus to improve stuttering by providing delayed auditory feedback in the form of mechanical vibrations. The device consists of a control mechanism suitable for inserting into a pocket, a separate microphone, and an electromechanical transducer attached to the neck of the user to provide the delayed feedback.
U.S. Pat. No. 4,464,119 to Vildgrube et al. discloses a device for correcting speech using DAF. The device is composed of a separate microphone, an analog control circuit, and an earphone. Similarly, Chinese Patent No. CN 1087742 to Ren, Zhiqiang, entitled, The Method of Delaying Analog Signals and The Stammer Curing Instrument Using This Method, proposes a DAF treatment device employing a plurality of storage mediums such as a recording magnetic head and a reading magnetic head.
Other treatment for stuttering continues to be uncertain, with many of the devices and therapies demanding excessive time in speech laboratories with specialized therapists and equipment or which treat stuttering by slowing down speech to make the speech sound more fluent. Conventional DAF treatment has utilized devices that are relatively large and stationary and not suited for prolonged use. In addition, drug therapies also have proven to be of somewhat limited efficacy and can potentially result in undesired side effects.